1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus wherein a rotary scanner deflects a light beam emitted from a light source to scan a light receiving surface line by line.
2. Description of Related Art
A light beam scanning system provided with a polygonal mirror has been developed into various types to be used as an image writing device. The polygonal mirror is driven by a motor to rotate at a constant speed. If image writing is performed while the motor is in a startup state, wherein the rotation of the motor is unstable, and while the motor rotates at not a specified speed, image disorder will be caused. Also, if the motor cannot keep the constant speed even in a steady state, the picture quality such as the image width will not meet the requirement.
Motors are classified into brush motors and brushless motors, and a brushless motor is used to drive the polygonal mirror. Brushless motors are expensive, but they have, in a motor control circuit, check means which judges whether the motor comes to a steady rotation state and generates a lock signal when the rotation of the motor comes to the steady state. If image writing is started after the lock signal, image disorder can be avoided. Further, brushless motors are good in the accuracy of rotation.
Brush motors are inexpensive. However, they do not have such check means, and the control circuit cannot check whether the rotation of the motor is normal. Moreover, brush motors vary in the rotating speed in a steady state depending on the individualities, the temperature of the circumstances, the heating condition of the coil and the driving circuit parts, etc, and it is difficult to regulate the rotating speed. Therefore, brush motors have not been used for driving a polygonal mirror.